The present invent ion relates to an antenna system having at least one LMK (Long-Medium-Short) radio aerial antenna which is arranged on a window or another non-conducting part of a motor vehicle and has an antenna capacitance Ca, and having an antenna booster for the LMK reception.
Such antenna systems are used in particular on the rear window of a motor vehicle. However, they can of course be used on other windows such as side windows, the windshield or other non-conducting parts of motor vehicles such as spoilers.
An antenna system is described in European Published Patent Application No. 0 155 647. This known antenna system, which obviously is conceived exclusively for use on the rear window of motor vehicles, is arranged on an area not covered by the heating field. The LMK antenna has a planar extension.
It can be inferred from the teaching of European Published Patent Application No. 0 155 647 that the size of the antenna area must be in a specific relationship to the size of the available area, in order to receive a maximum input signal for the antenna booster. It is specifically explained that, in order for the reception to become optimum, it is also absolutely necessary that the signal voltage be maximum at the input of the LMK booster.
In this context, it is apparently assumed that the noise voltage at the input terminal of the LMK booster is independent of the source impedance.
However, in practice it has turned out that when using an antenna system according to European Published Patent Application No. 0 155 647, the reception is not optimal under certain conditions.
According to the present invention, it has been recognized that this is attributable to the fact that a maximization of the signal voltage at the input terminal of the antenna booster does not deliver an optimal result under all operating conditions, since the assumption, apparently made in European Published Patent Application No. 0 155 647, about the independence of the noise voltage from the source impedance is incorrect. Namely, the noise voltage of an FET (field-effect transistor) customarily used as input transistor in the LMK booster actually increases perceptibly, at least in the long-wave range and in the xe2x80x9clowxe2x80x9d medium-wave range, when a conductive surface acting as antenna is used as the signal source.
The reason for this is that the conductive surface acts as a capacitor whose impedance increases as the frequency falls. All transistors exhibit minimal noise only in response to a quite specific source impedance. Their noise increases in the case of impedance deviating therefrom. Specifically, this means that although the antenna known from European Published Patent Application No. 0 155 647 may be optimally matched in the upper medium-wave range and in the short-wave range (also known as the upper LMK range), losses in the reception quality are accepted in the lower medium-wave range and in the long-wave range (lower LMK range). This is also of particular disadvantage since the signal-to-noise ratio of the antenna is markedly poorer anyway in the lower LMK range and, above all, in the long-wave range, than in the upper LMK range.
An object of the present invention is to further develop an antenna system in such a way that optimum reception is yielded under virtually all operating conditions and frequencies.
In this context, the present invention starts out from the root idea of carrying out the matching between the LMK antenna and the antenna booster in such a way that an optimal signal-to-noise ratio results in the lower LMK range.
To that end, according to the present invention, the distance of the antenna conductor(s) to the parts surrounding the LMK antenna is selected in such a way that the value of the harmful capacitance Ca of the LMK antenna, which is essentially formed by the capacitance between the antenna conductors and the conductive parts surrounding the antenna, is selected to be greater than the value at which the maximum antenna output voltage results, and that the value of the harmful capacitance Ca is selected such that, by appropriate selection of the source impedance of the antenna booster, the signal-to-noise ratio is optimized not in the upper, but rather in the lower LMK range.
Thus, the present invention breaks with the usual procedure, according to which harmful or stray capacitance is minimized to the greatest extent possible. On the contrary, according to the present invention, a comparatively large harmful capacitance is deliberately xe2x80x9cadjustedxe2x80x9d, by which the source capacitance of the input transistor is increased in such a way that, although the input signal is not maximal, the signal-to-noise ratio is probably optimal.
This matching according to the present invention can be achieved, for example, by selecting conductive area A acting as antenna to be larger than is described in European Published Patent Application No. 0 155 647. Since according to the present invention, area A is enlarged compared to the related art, the capacitance of the antenna increases specific to the electromagnetic field.
At the same time, the harmful capacitance increases because of the reduced distance to the conductive parts such as metallic body parts, the heating wires of the rear window, etc. surrounding the antenna conductors. Therefore, given the same frequency f of the electromagnetic signal, the amount of the capacitive portion Zk of the source impedance is smaller. Consequently, the amount Z of the source impedance decreases.
It may be that the signal voltage at the booster input also decreases, however, the input noise of the transistor utilized is also very markedly reduced in the lower LMK range at the same time. In the long-wave range (with small signal frequency f) and in the lower medium-wave range, this measure leads to an improvement in the signal-to-noise ratio for virtually all common transistors.